The broad, long-term objectives of this proposal are to determine the specific function and macromolecular structure of Kv1.5 (a voltage-dependent K+ channel alpha subunit possibly involved in repolarization of the cardiac action potential). This work is based on the hypothesis that the Kv1.5 alpha subunit is a component of a K+ channel involved in phase repolarization of the cardiac action potential (and hence functions as a receptor of anti-arrhythmic drugs with Class III action). This work is a first step in the long process of dissecting the proteins that mediate repolarizing outward current in the heart. The Specific Aims are 1) compare the electrophysiological characteristics of delayed-rectifier K+ currents in wild-type cardiac myocytes with those from mutant animals lacking a functional Kv1.5 gene, 2) compare ambulatory electrocardiograms in mice from three genetic backgrounds (wild-type, Kv1.5-null, heterozygotic) at baseline and after exposure to drugs that modulate myocyte repolarization, and 3) determine whether the absence of Kv1.5 in vivo modulates protein and transcript levels of other alpha and/or beta subunits. Increasingly, disorders of repolarization are causally implicated in many malignant cardiac arrhythmias. These arrhythmias underlie sudden cardiac death which is a major killer of Americans today causing an estimated 400,000 deaths per year. Increasingly, we treat atrial fibrillation the commonest arrhythmia, afflicting 1 percent of persons older than 60 years old-with repolarization-active anti-arrhythmic agents. The structural basis of repolarizing current in heart is presently poorly characterized. However, the ion channels that mediate repolarizing K+ current serve as receptors for anti-arrhythmic drugs with Class III activity, one of our major weapons against these devastating arrhythmias. The research design centers on comparing physiologic parameters and biochemical features of wild-type mice with mice deficient in Kv1.5. The methods to be used are biochemical and immunological (Western blots, immunoprecipitation) and electrophysiological (patch clamping, ambulatory electrocardiography).